This invention relates to the reception and demodulation of communications signals, and more particularly, to the reception of such signals transmitted with selectable symbol rates.
In a communications system, symbols (also sometimes called "data") is formatted onto a carrier signal and transmitted by a transmitter. After the signal travels through some intervening medium, it is received and decoded by the receiver. Ideally, the waveform of the symbols would remain unchanged during the communications process. In practice, however, the waveform is distorted and corrupted by its passage both through the electronic circuitry of the transmitter and the receiver, and through the medium. An important feature of the receiver is the processing of the received signal to determine the actual content of the symbol stream even though the transmitting signal has become distorted and corrupted during the transmission and reception process.
For example, in a typical satellite communications system a symbol stream is created at one location on the earth, encoded onto a radio signal, and transmitted to a satellite in synchronous orbit above the earth. The satellite retransmits the received signal to another location on the earth, where it is received and demodulated. The data-carrying symbol stream passes through several electronic systems, thousands of miles of free space, and twice through the atmosphere, and in all of these portions of the transmission it is subject to external interference and distortions.
Historically, the signal has been transmitted and processed entirely by analog techniques. More recently, digital signal processing techniques are being adopted because they permit more precise determination of the data content of the signal. In digital signal processing, the receiver has a conventional tuner that receives and down converts the signal. The receiver thereafter samples the received analog signal to form a digital pulse train or signal. The digitized signal is further processed to extract the data content of the symbol stream.
This known approach works well for the condition that the transmitted signal has a fixed symbol rate known to the receiver, which permits the receiver to be configured for the characteristics of the known transmitted signal. In other instances, however, it is desirable to vary the symbol rate of the transmitter in an arbitrary manner. For example, a single satellite channel may be used to carry many different types of data signals, some of which are transmitted at a high symbol rate and some of which are transmitted at a low symbol rate. In another example, if the satellite channel carries a digitized video signal, it may be desirable to vary the symbol rate depending upon the type of programming being carried. A video feed of a conference could be transmitted at a lower symbol rate than a video feed of a sports event, for example, due to the differences in the speed of the action. The lower the symbol rate of the signal, the more different types of data that could be carried by a single satellite channel. The ability to carry larger amounts of data with fixed satellite channel capability becomes increasingly important as more video programs and other information are available for transmission.
Several problems arise in complex communications systems having multiple channels, where the symbol rate is variable in each channel. As the symbol rate of the signal in a channel changes, the sampling rate of the sampler in the receiver must change in order to satisfy the Nyquist sampling criterion. The sampler is normally synchronized to a clock signal, which changes to permit the sampler to be varied to an arbitrary sampling rate. However, for other reasons it is strongly preferred not to change the clock rate in an arbitrary fashion so as to accommodate changes in the symbol rate. A second problem is that it is difficult to achieve initial acquisition of the signal of the received information, when the channel is first activated, and to follow changes in the transmission parameters as they are made during transmissions, unless these parameters are otherwise made known to the receiver.
There is a need for a digital receiver system that is operable at controllably variable rates, particularly in a multichannel communications system. The present invention fulfills this need, and further provides related advantages.